1. Field of the Invention
This invention relates to brazing of metal parts; and more particularly, to a homogeneous, ductile iron-chromium-based brazing material useful in brazing stainless steels, and a method for brazing stainless steel components to form articles of manufacture that reduces the propensity of nickel to leach from such articles in water.
2. Description of the Prior Art
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a filler metal that has a melting point lower than that of the metal parts to be joined together is interposed between the metal parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the filler metal. Upon cooling, a strong, leak-tight joint is formed. The assembled parts may either constitute a finished article of manufacture or they may form a sub-component for use in a further manufacturing operation.
The selection of a particular brazing filler metal for a specific application depends on a variety of factors, including requirements related to the components to be joined and to the conditions under which the assembly ultimately must operate.
One basic consideration is temperature. Brazing filler metals are characterized by their solidus and liquidus temperatures. The term xe2x80x9csolidusxe2x80x9d refers to the highest temperature at which a metal or alloy is completely solid, and the term xe2x80x9cliquidusxe2x80x9d refers to the lowest temperature at which the metal or alloy is completely liquid. In any brazing process, the brazing filler metal must possess a solidus temperature that is high enough to provide the brazed assembly with adequate integrity to meet the desired service requirements and yet have a liquidus that is low enough to be compatible with the temperature capabilities of the parts being joined.
Another consideration is corrosion resistance. Many brazed assemblies must operate under environmental conditions that are conducive to corrosion, especially in the vicinity of the brazement. The propensity of a given system to corrode is strongly influenced by the gases or liquids to which the system is exposed and by typical operating temperatures.
One class of devices which are frequently assembled using brazing as a joining technique is heat exchangers. These devices are known in a variety of configurations. Generally stated, heat exchangers allow heat to be transferred across an interface that separates one circulating fluid from another circulating fluid. It is generally essential that the fluids, either of which can be gaseous or liquid, be kept separate. Hence, it is critical that brazed joints which define, at least in part, the interface maintain structural integrity under a full range of operating conditions and for a prolonged service life.
One field of use wherein heat exchangers find utility is in the processing of materials which are ultimately intended for human ingestion and consumption. These include foodstuffs, as well as fluids such as water, beverages, juices, and the like. The metallic materials used for the construction of heat exchangers appointed for such applications are of critical importance. They not only need to provide excellent operative characteristics with regard to heat transfer, but also must be compatible with the substances to which they are exposed. One particular concern is the requirement that there be no undesired leaching or elution of any elemental or molecular component species of the materials of construction that is harmful or adds undesirable taste to the fluids. If a harmful species or an undesirable taste is present, then it is imperative that any leaching of causative materials be minimized. Frequently, local governmental or regulatory authorities have established maximum amounts of materials, such as metal ions, which may be permitted to leach into fluids passing therethrough. The standard is ordinarily expressed as a maximum amount of leachate that may be present per unit volume of the fluid processed. Ideally, the materials incorporated in heat exchangers (including brazing filler metals) and the associated manufacturing methods result in a device that meets or exceeds applicable regulatory standards under foreseeable operating conditions.
Heat exchangers of the xe2x80x9cshell-and-tube,xe2x80x9d xe2x80x9cplate/plate,xe2x80x9d and xe2x80x9cplate/finxe2x80x9d types are most usually encountered. In the first configuration, a larger diameter housing typically referred to as a xe2x80x9cshellxe2x80x9d encompasses one or more small diameter tubes or pipes. According to this configuration, a first fluid (i.e., liquid, gas) passes through the shell and about the exterior of the tubes while simultaneously, a second fluid (liquid, gas) passes through the interior of the tubes. While no physical contact is permitted between the first and second fluids, heat transfer occurs across the walls of the tubes from the hotter fluid to the cooler fluid. In plate/plate and plate/fin type heat exchangers, again a physical member, namely one or more plates separate a first fluid from a second fluid while heat transfer occurs across the plate. In these types of heat exchanger (as well as in other assemblies), metals are most commonly used due to their high strength and good heat transfer characteristics. Typically, the individual parts, which are used to make up these types of heat exchangers, are joined by brazing. It is imperative that the heat exchanger maintain its physical integrity and the isolation of the fluids from each other and the outside world. In addition, the heat exchanger and the joints that secure its internal components must be resistant to any potential detrimental effects which might result from contact with one or both of the fluids.
To minimize this undesired technical effect, the materials of construction for heat exchangers, particularly those used for foodstuffs, need to be very carefully selected. Stainless steels, which contain up to about 20% Ni, are very commonly encountered, for they exhibit desirable properties including low leaching rates into fluids or gases, and generally good corrosion resistance. However, brazing manufacturing processes carried out at high temperatures may also adversely affect the propensity of the stainless steels to leach. Previously, elemental copper was used as a brazing filler metal as such featured low leaching of nickel into fluids, especially water. However, the corrosion resistance of heat exchangers having components brazed using copper as brazing filler metal is poor. Typically these heat exchangers required frequent replacement, resulting in significant costs for the replacement device and the associated labor, as well as economic losses resulting from manufacturing downtime. To improve corrosion resistance, it was recently found that brazing filler metals with compositions based primarily on nickel and chromium (xe2x80x9cNi/Crxe2x80x9d) could be employed to join stainless steel parts used in such assemblies. Unfortunately, it was also found that when such Ni/Cr-based brazing filler metals were used, an undesirably high amount of nickel often leached into water or other fluids flowing through these assemblies.
Inasmuch as such Ni/Cr-based brazing filler metals include a significant proportion of nickel, they are believed to be the source of the undesired nickel leachate. For this reason, use of Ni/Cr-based brazing filler metals should be avoided in applications where nickel leaching into a fluid presents a concern, as is the case when materials are appointed to be used for human ingestion or consumption. Not surprisingly, governmental regulations in some countries have imposed strict limitations on the amount of nickel which may be leached into these fluids. It is to one or more of these technical needs that the present invention is directed.
The present invention provides a method for fabricating heat exchangers and other articles of manufacture by brazing components thereof with an iron/chromium brazing filler metal. Brazed assemblies advantageously exhibit good general corrosion resistance and low rates of leaching of nickel into fluids passing through either side of the heat exchanger. As a result, the heat exchanger is highly suited for exposure to items intended for ingestion by humans or animals.
In a first aspect, the present invention provides a method for the manufacture of assemblies, especially those which include parts made of stainless steels. These assemblies comprise parts joined using iron/chromium-based brazing filler metals. When manufactured, the assemblies are characterized by general corrosion resistance and by low leaching rates of nickel. The method comprises the steps of: (a) juxtaposing at least two parts to define one or more joints therebetween; (b) supplying to the one or more joints an iron/chromium brazing filler metal; (c) heating the juxtaposed parts and the brazing filler metal under appropriate conditions in order to cause the melting of the brazing filler metal; and (d) cooling the melted brazing filler metal to produce a brazed joint. The heating and cooling steps are preferably carried out either in a protective gas atmosphere or in vacuum.
In a second aspect, the present invention relates to a method for the manufacture of a heat exchanger or other manufactured assembly which method includes a brazing step in which the alloys of the present invention are used, as well as a post-brazing conditioning step for treating the brazed heat exchanger. The conditioning step comprises exposing the brazed joints to an elevated temperature, in an oxygen-containing atmosphere, for a sufficient time to form a protective oxide layer to substantially reduce the amount of nickel leaching into water or other fluids contacting the brazed joints of the brazed assembly. Preferably the conditioning step is carried out in air and at a temperature ranging from about 150xc2x0 to 600xc2x0 C. More preferably, the conditioning is done at a temperature ranging from about 150xc2x0 to 400xc2x0 C. The manufactured heat exchangers are characterized by general corrosion resistance and by reduced leaching rates of nickel into fluids passed through the heat exchanger.
In a third aspect of the invention, there are provided manufactured assemblies which comprise parts joined by iron/chromium-based brazing filler metals, wherein the assemblies are characterized by general corrosion resistance and by reduced nickel leaching rates.
In a fourth aspect, there is provided an iron/chromium-based brazing filler metal alloy. Preferably the iron/chromium-based brazing filler metal consists essentially of a composition having the formula FeaCrbCocNidMoeWfBgSih wherein the subscripts xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d, xe2x80x9ccxe2x80x9d, xe2x80x9cdxe2x80x9d, xe2x80x9cexe2x80x9d, xe2x80x9cfxe2x80x9d, xe2x80x9cgxe2x80x9d, and xe2x80x9chxe2x80x9d are all in atom percent and wherein, xe2x80x9cbxe2x80x9d is between about 5 and 20, xe2x80x9ccxe2x80x9d is between 0 and about 30, xe2x80x9cdxe2x80x9d is between 0 and about 20, xe2x80x9cexe2x80x9d is between 0 and about 5, xe2x80x9cfxe2x80x9d is between 0 and about 5, xe2x80x9cgxe2x80x9d is between about 8 and 15, xe2x80x9chxe2x80x9d is between about 8 and 15, the sum xe2x80x9caxe2x80x9d+xe2x80x9cbxe2x80x9d+xe2x80x9ccxe2x80x9d+xe2x80x9cdxe2x80x9d+xe2x80x9cexe2x80x9d+xe2x80x9cfxe2x80x9d+xe2x80x9cgxe2x80x9d+xe2x80x9chxe2x80x9d=100, and incidental impurities and other elements are optionally present in an amount up to about 1 percent by weight of the total composition. The preferred iron/chromium brazing filler metal is especially suited for fabricating heat exchangers and other assemblies of the invention which exhibit low Ni leaching rates. It is preferred that the iron/chromium brazing filler metal be prepared in the form of a homogeneous, ductile ribbon or strip.
The alloys of the present invention contain substantial amounts of boron and silicon, which are present in the solid state in the form of hard and brittle borides and silicides. Accordingly, the alloys of the invention are especially suited for fabrication into flexible thin foil by rapid solidification techniques. Foil produced in this manner is composed of a metastable material having at least 50% glassy structure and a thickness ranging from about 0.0007 to 0.002 inches (18-50 xcexcm). Use of a thin flexible and homogeneous foil as filler metal is especially beneficial for brazements wherein the mating surfaces have wide areas with narrow clearances and for brazing joints having complex shapes. The alloys of the invention can also be produced in powder form by gas or water atomization of the alloy or by mechanical comminution of a foil composed thereof. Other methods, such as rolling, casting, and other powder metallurgical techniques can be also be used to prepare these alloys.
Further aspects and features of the invention will become more apparent from the following description.